text: The composition of corium depends on the design type of the reactor, and specifically on the materials used in the control rods, coolant and reactor vessel structural materials. There are differences between pressurized water reactor (PWR) and boiling water reactor (BWR) coriums.
  In contact with water, hot boron carbide from BWR reactor control rods forms first boron oxide and methane, then boric acid. Boron may also continue to contribute to reactions by the boric acid in an emergency coolant.
  Zirconium from zircaloy, together with other metals, reacts with water and produces zirconium dioxide and hydrogen. The production of hydrogen is a major danger in reactor accidents. The balance between oxidizing and reducing chemical environments and the proportion of water and hydrogen influences the formation of chemical compounds. Variations in the volatility of core materials influence the ratio of released elements to unreleased elements. For instance, in an inert atmosphere, the silver-indium-cadmium alloy of control rods releases almost only cadmium. In the presence of water, the indium forms volatile indium(I) oxide and indium(I) hydroxide, which can evaporate and form an aerosol of indium(III) oxide. The indium oxidation is inhibited by a hydrogen-rich atmosphere, resulting in lower indium releases. Caesium and iodine from the fission products can react to produce volatile caesium iodide, which condenses as an aerosol.
  During a meltdown, the temperature of the fuel rods increases and they can deform, in the case of zircaloy cladding, above . If the reactor pressure is low, the pressure inside the fuel rods ruptures the control rod cladding. High-pressure conditions push the cladding onto the fuel pellets, promoting formation of uranium dioxide–zirconium eutectic with a melting point of . An exothermic reaction occurs between steam and zirconium, which may produce enough heat to be self-sustaining without the contribution of decay heat from radioactivity. Hydrogen is released in an amount of about  of hydrogen (at normal temperature/pressure) per kilogram of zircaloy oxidized. Hydrogen embrittlement may also occur in the reactor materials and volatile fission products can be released from damaged fuel rods. Between , the silver-indium-cadmium alloy of control rods melts, together with the evaporation of control rod cladding. At , the cladding oxides melt and begin to flow. At  the uranium oxide fuel rods melt and the reactor core structure and geometry collapses. This can occur at lower temperatures if a eutectic uranium oxide-zirconium composition is formed. At that point, the corium is virtually free of volatile constituents that are not chemically bound, resulting in correspondingly lower heat production (by about 25%) as the volatile isotopes relocate.
  The temperature of corium can be as high as  in the first hours after the meltdown, potentially reaching over . A large amount of heat can be released by reaction of metals (particularly zirconium) in corium with water. Flooding of the corium mass with water, or the drop of molten corium mass into a water pool, may result in a temperature spike and production of large amounts of hydrogen, which can result in a pressure spike in the containment vessel. The steam explosion resulting from such sudden corium-water contact can disperse the materials and form projectiles that may damage the containment vessel by impact. Subsequent pressure spikes can be caused by combustion of the released hydrogen. Detonation risks can be reduced by the use of catalytic hydrogen recombiners.
  Brief re-criticality (resumption of neutron-induced fission) in parts of the corium is a theoretical but remote possibility with commercial reactor fuel, due to low enrichment and the loss of moderator. This condition could be detected by presence of short life fission products long after the meltdown, in amounts that are too high to remain from the pre-meltdown reactor or be due to spontaneous fission of reactor-created actinides.